The turbine section of gas turbine engines typically comprises multiple sets or stages of stationary blades, known as nozzles or vanes, and moving blades, known as rotor blades or buckets. FIG. 1 illustrates a typical rotor blade 100 found in the first stage of the turbine section, which is the section immediately adjacent the combustion section of the gas turbine and thus is in the region of the turbine section that is exposed to the highest temperatures. Known problems with such blades 100 include premature cracking at the root trailing edge 104, and cracking and/or delamination of a thermal barrier coating (“TBC”) in the platform region 106 due to the heat stresses in this region of the blade. As shown in FIG. 1, the cracking 104 typically commences at a root trailing edge cooling channel 110a located on a trailing edge 112 of an airfoil 102 of the blade 100 adjacent the platform 108. This root trailing edge cooling channel 110a is particularly vulnerable to thermal mechanical fatigue (“TMF”) because of excessive localized stress that occurs during start-stop cycles and creep damage that occurs under moderate operating temperatures, i.e., during periods of base load operation. Because the root trailing edge cooling channel 110a is affected by both mechanisms, premature cracking 104 has been reported within the first hot gas path inspection cycle. If the cracking 104 is severe enough, it can force early retirement of the blade 100. As also shown in FIG. 1, in some cases the cracking in the platform region 106 is so severe that it results in breakage and separation of a substantial portion of the platform on the pressure side of the blade 100, leading to the early retirement of the blade. In order to prevent early retirement and to extend blade operational lifetime, various approaches have been proposed.
The principal damage at the root trailing edge cooling channel 110a can be consequence of the combination of mechanical stress due to centrifugal load and thermal stress that results from the significant temperature gradient present at the root trailing edge cooling channel 110a. The initial damage is generally relatively confined, i.e., the cracking 104 appears localized. This suggests that the blade 100 might be salvaged if the confined damage is removed. In order to restore the structural integrity of the blade 100 however, it is desirable to remove all of the original cracking 104. In other words, any removal of material from the trailing edge 112 should be of sufficient depth to eliminate the cracking 104. However, it is undesirable to remove too much material as this can reduce the strength of the blade 100 to the degree that new cracking 104 might form even more quickly.
In a previously proposed solution, an undercut is machined into the blade platform. An example of such an undercut can be found in FIG. 2, which illustrates an elliptical-shaped groove 150 which extends from the concave side of platform to the trailing edge side of the platform. This proposed solution purports to reduce the total stress level in the region of high stress, for example proximate the cooling channel closest to the platform in the root portion of the trailing edge.
The goal of the undercut approach is to alleviate both the mechanical stress and the thermal stress by relaxing the rigidity of that juncture where the airfoil and platform join. This approach has been implemented on both turbine and compressor blades, both as a field repair and a design modification. If a stress reduction is achieved, the concern is whether the undercut results in a high stress within the grooved region where material is removed. In other words, the success of the strategy turns on whether a balance can be achieved without creating a new area of stress within the blade.
There are two primary concerns raised with platform undercuts. First, whether the undercut will be effective in reducing the stress. Second, whether the stress produced in the undercut will be so high that it offsets the benefit of the undercut. The problem with prior undercut solutions is that they have had difficulty striking that balance. It is desired to have a solution which reduces the stress at the trailing edge and/or in the platform, but minimizes the stress in the region of the undercut. The present invention seeks to solve this problem, among others.